Extreme ultraviolet radiation, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including radiation at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates such as silicon wafers.
Methods for generating EUV radiation include converting a target material to a plasma state. The target material preferably includes at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV portion of the electromagnetic spectrum. The target material can be solid, liquid, or gas. In one such method, often termed laser produced plasma (“LPP”), the required plasma can be produced by using a laser beam to irradiate a target material having the required line-emitting element.
One LPP technique involves generating a stream of target material droplets and irradiating at least some of the droplets with one or more laser radiation pulses. Such LPP sources generate EUV radiation by coupling laser energy into a target material having at least one EUV emitting element, creating a highly ionized plasma with electron temperatures of several 10's of eV.
For this process, the plasma is typically produced in a sealed vessel, e.g., a vacuum chamber, and the resultant EUV radiation is monitored using various types of metrology equipment. In addition to generating EUV radiation, the processes used to generate plasma also typically generate undesirable by-products in the plasma chamber which can include out-of-band radiation, high energy ions and debris, e.g., atoms and/or clumps/microdroplets of residual target material.
The energetic radiation is emitted from the plasma in all directions. In one common arrangement, a near-normal-incidence mirror (often termed a “collector mirror” or simply a “collector”) is positioned to collect, direct, and, in some arrangements, focus at least a portion of the radiation to an intermediate location. The collected radiation may then be relayed from the intermediate location to a set of optics, a reticle, detectors and ultimately to a silicon wafer.
In the EUV portion of the spectrum it is generally regarded as necessary to use reflective optics for the optical elements in the system including the collector, illuminator, and projection optics box. These reflective optics may be implemented as normal incidence optics as mentioned or as grazing incidence optics. At the wavelengths involved, the collector is advantageously implemented as a multi-layer mirror (“MLM”). As its name implies, this MLM is generally made up of alternating layers of material (the MLM stack) over a foundation or substrate. System optics may also be configured as a coated optical element even if it is not implemented as an MLM.
The optical element must be placed within the vessel with the plasma to collect and redirect the EUV radiation. The environment within the chamber is inimical to the optical element and so limits its useful lifetime, for example, by degrading its reflectivity. An optical element within the environment may be exposed to high energy ions or particles of target material. The particles of target material, which are essentially debris from the laser vaporization process, can contaminate the optical element's exposed surface. Particles of target material can also cause physical damage to and localized heating of the MLM surface.
In some systems H2 gas at pressures in the range of 0.5 to 3 mbar is used in the vacuum chamber as a buffer gas for debris mitigation. In the absence of a gas, at vacuum pressure, it would be difficult to protect the collector adequately from target material debris ejected from the irradiation region. Hydrogen is relatively transparent to EUV radiation having a wavelength of about 13.5 nm and so is preferred to other candidate gases such as He, Ar, or other gases which exhibit a higher absorption at about 13.5 nm.
H2 gas is introduced into the vacuum chamber to slow down the energetic debris (ions, atoms, and clusters) of target material created by the plasma. The debris is slowed down by collisions with the gas molecules. For this purpose a flow of H2 gas is used which may also be counter to the debris trajectory and away from the collector. This serves to reduce the damage of deposition, implantation, and sputtering target material on the optical coating of the collector.
The process of generating EUV light may also cause target material to be deposited on the walls of the vessel. Minimizing target material deposition on the vessel walls is important for achieving an acceptably long lifetime of EUV sources placed in production. Also, maintaining the direction of target material flux from the irradiation site and directionality of power dissipation into the buffer gas is important for ensuring that the waste target material mitigation system works as intended and can acceptably manage by-products associated with vaporization of the target material.